Intervertebral prostheses are used for replacing intervertebral disks. They comprise two cover plates whose outer surfaces are designed for connection to adjacent vertebral bodies, and of an articulation device enclosed by the cover plates. In a known prosthesis type (WO 01/01893, FR-A-2718635), the upper cover plate forms a concavely spherical articulation surface on its inner side, which cooperates with the convexly spherical top surface of a prosthesis core of polyethylene in order to form an articulation. The flat underside and the edge of the core are received with matching fit in a seat which is formed by the lower cover plate. This seat comprises a plane bottom surface and a vertical edge which surrounds the latter on three sides (laterally and dorsally). On the sides, the edge is formed by two undercut edge ridges which extend substantially in the anterior-posterior (hereinafter “AP”) direction and to which complementary projecting ridges or grooves on the edge of the prosthesis core correspond. On the ventral side, the edge of the cover plate is open so that the prosthesis core can be pushed like a drawer into the edge of the cover plate, in a movement extending in the AP direction. In the pushed-in state, the prosthesis core is secured against lifting by means of the interaction of the projecting ridges and grooves of the cover plate and of the prosthesis core. In particular, it cannot move dorsally from the intended position toward the spinal cord when the cover plates spread open dorsally during a flexion movement.
There are known intervertebral prostheses in which the edge of the lower cover plate is also closed ventrally (EP-B-471821, U.S. Pat. No. 5,425,773). However, this has the disadvantage that the prosthesis core either cannot be inserted between the cover plates after they have been implanted or is not secured against lifting in the seat of the lower cover plate. By contrast, the invention relates to the type of prosthesis in which the seat of the lower cover plate is designed as a ventrally open push-in guide for the prosthesis core, so that this can be inserted into the prosthesis after the cover plates have been implanted.
To ensure that the prosthesis core cannot escape ventrally from the seat, a locking limit stop is provided in the known prostheses of this type (FR-A-2 718 635, WO 01/01893). This locking limit stop consists of interacting projections and recesses in the underside of the prosthesis core, on the one hand, and in the bottom surface of the seat, on the other hand. To ensure that these can come into engagement with one another when the prosthesis core is pushed into the seat, the seat which is made of resilient plastic material must deform elastically before these elements lock together. The disadvantage of this is that the interlocking is in principle unsafe, because it can be released again by a corresponding elastic deformation of the prosthesis core. It is of course possible to improve the locking safety by giving the prosthesis core the greatest possible resistance to deformation. However, this makes it difficult for the operating surgeon to insert the prosthesis core into the seat. It can also happen that, for random reasons of which the operating surgeon remains unaware, the lock elements do not attain the lokking position, or do so incompletely, for example because a foreign body has been left in the locking recess or because a chance obstacle has meant that the prosthesis core has not been pushed far enough into the seat.